When I move my index finger to "<" my right pinky is already on shift. I don't think pressing shift to change Z levels is a big deal either. Pressing shift to navigate the menus is a little annoying.
Previously I had a slightly different [physical] keyboard layout, and now I'm not sure, probably I'll be using = and ], or ; and . for secondary scroll - again because they're on the right, don't require Shift and better correspond to "up" and "down".
Using keys that are above/below each other is a good idea. Having them left/right of each other is a little confusing. The = and ] might be the best option for scrolling. It's not too different from the existing scroll keys, it doesn't conflict with existing keys, and when holding down shift, it still doesn't conflict with existing keys so you can use Max's idea of holding down "shift" to turn the scroll keys into page up/down keys.
Second best choice is the ' and / keys, but since / conflicts with the help key with shift held down, it couldn't use Max's page up/down idea. So maybe those using those for Z levels, like you said, is best use for them.
With ; and . for scrolling, the . is used to advance time, so that might not be a good choice.